Allow JCR clients to navigate the nodes exposed by an external source

Creating a projection via the FederationManager requires knowing the path of the external node within the external source. This enhancement is to provide a way for JCR clients to explore the external source content before any projections are created.

One possibility is to reuse the JCR API by exposing the external source as a "non-persistent" javax.jcr.Workspace and javax.jcr.Session. This may be a significant amount of work and require a lot of extra content/functionality/behavior than is desirable. Perhaps it might be possible to expose the content via a top-level javax.jcr.Node for the external source.